拓扑绝缘体Bi $_2$ Se $_3$表面态的太赫兹响应和巨大克尔旋转
THz response and colossal Kerr rotation from the surface states of the topological insulator Bi$_2$Se$_3$
摘要：
有序的物质状态通常根据它们的对称性被破坏来分类。随着铁磁体中自旋的有序化或液体冻结成固体，对称性的丧失将有序状态与无序状态区分开来。相反，拓扑状态通过编码在其量子力学波函数中的特定\textit{topological}性质来区分。通常情况下，这些属性的结果是在样本的边界上存在鲁棒的“拓扑保护”状态。量子霍尔效应（QHE）的边缘态是经典的例子。在过去的几年里，人们意识到另一类这样的拓扑物质可能存在于具有大自旋轨道相互作用的3D带绝缘体中。这些所谓的拓扑绝缘体预计主机强大的表面状态，表现出一些有趣的属性，包括自旋螺旋，免疫力反向散射，和弱\textit{anti} -本地化。有一些不寻常的现象与这些表面状态的预测，包括邻近效应引起的奇异超导状态与马约拉纳费米子绑定到一个漩涡和“轴子”的电磁响应，和应用的建议，如它们在太赫兹设备中的使用。在这封信中，我们报告了一个前所未有的大旋转的观察线偏振光反射的偏振面从薄膜的拓扑绝缘体Bi $_2$ Se $_3$在磁场中。这种克尔旋转可以大到65 $^\circ$，并且几乎完全是由于表面状态。我们的研究结果是拓扑保护的表面状态的内在响应的证据，并提供了一个基准预测这些材料的大磁电效应。
英文摘要：
Ordered states of matter are typically categorized by their broken symmetries. With the ordering of spins in a ferromagnet or the freezing of a liquid into a solid, the loss of symmetry distinguishes the ordered state from the disordered one. In contrast, topological states are distinguished by specific \textit{topological} properties that are encoded in their quantum mechanical wavefunctions. Frequently, a consequence of these properties is that there are robust "topologically protected" states on the sample's boundaries. The edge states of the quantum Hall effect (QHE) are the classic example. In the last few years, it was realized that another class of such topological matter may exist in 3D band insulators with large spin-orbit interaction. These so-called topological insulators are predicted to host robust surface states, which exhibit a number of interesting properties including spin helicity, immunity to back-scattering, and weak \textit{anti}--localization. There are predictions of a number of unusual phenomena associated with these surface states, including a proximity-effect-induced exotic superconducting state with Majorana fermions bound to a vortex and an `axion' electromagnetic response, and proposals for applications, such as their use in terahertz devices. In this Letter we report the observation of an unprecedentedly large rotation of the polarization plane of linearly polarized light reflected from thin films of the topological insulator Bi$_2$Se$_3$ in magnetic fields. This Kerr rotation can be as large as 65$^\circ$ and is due almost entirely to the surface states. Our results are evidence for the intrinsic response of the topologically protected surface states and provide a benchmark for the large magneto-electric effect predicted for these materials.
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